1. Field of the Invention
The present invention relates to a process for preparing salts of methallylsulfonic acid by sulfonation of isobutene by means of Lewis base-sulfur trioxide complexes and subsequent neutralization.
2. Description of the Background
Salts of methallylsulfonic acid are used as comonomers in the synthesis of fiber intermediates, in particular for the modification of polyacrylonitriles. Furthermore, these salts are employed in the production of dispersions.
Salts of methallylsulfonic acid are prepared industrially by two different routes starting from isobutene:
In the first synthetic route, isobutene is reacted with chlorine to form the intermediate methallyl chloride. This compound reacts with sulfites to give the target products. For example, the reaction of methallyl chloride with sodium sulfite gives sodium methallylsulfonate and sodium chloride. Disadvantages of this synthesis are that the molar yield of target product is only 85% based on isobutene and inorganic chlorides are formed as coproducts and organic chlorine compounds are formed as by-products.
In the other synthetic route, sulfur trioxide is added onto isobutene and the resulting methallylsulfonic acid is subsequently reacted with a base to give the corresponding salt. Many different processes for this reaction sequence are known from the literature. Owing to the improved selectivity of the formation of methallylsulfonic acid, the sulfonation of isobutene is carried out using not sulfur trioxide but a complex of sulfur trioxide and a Lewis base.
Industrial processes for preparing salts of methallylsulfonic acid, in general the sodium salt, comprise the following steps:    (a) Preparation of a solution of a complex consisting of sulfur trioxide and a Lewis base and, if desired, a solvent    (b) Sulfonation of isobutene using the solution prepared in step (a)    (c) Conversion of the acids formed in step (b) into their salts    (d) Isolation and purification of the methallylsulfonate    (e) Recovery and purification of the complexing agent and, if desired, a solvent.
The processes employed in industry can be divided into two groups. In the first group, the preparation of the SO3-Lewis acid complex and its reaction with isobutene is carried out in an additional solvent. Customary solvents are ethylene glycol dimethyl ether and halogenated hydrocarbons such as methylene chloride, chloroform and 1,2-dichloroethane. A disadvantage of these processes is not only the Lewis acid but also a solvent have to be separated off and purified before recirculation to the process. Furthermore, losses of solvent result from chemical reactions. For example, halogenated hydrocarbons are hydrolyzed to a slight extent during the neutralization. This forms organic by-products which have to be separated off and an aqueous wastewater stream containing chlorides. These materials incur additional disposal costs.
In the second group of processes, no additional solvent is used. The liquid Lewis acid is used in excess and serves not only as complexing agent but also as solvent or suspension medium for the sulfur trioxide-Lewis acid complex.
DE 1 804 833 discloses a process for preparing salts of methallylsulfonic acid without use of an additional solvent. Here, isobutene is reacted with a complex of sulfur trioxide and an N,N-dialkyl-substituted amide of an aliphatic carboxylic acid or an N-alkylated lactam in an excess of the complexing agent as reaction medium at temperatures of from −20 to +60° C., using at least one mol of isobutene per mol of sulfur trioxide. The reaction mixture obtained is neutralized by means of an aqueous base. The neutralization step should be carried out in such a way that no secondary reactions occur. The only information given in respect of such a reaction procedure is the use of dilute caustic alkalis or weak alkalis such as sodium carbonate for neutralizing the reaction mixture. The aqueous phase is completely or partly evaporated and the target product is separated off. The yield of yellowish crude product, based on sulfur trioxide, is up to 97%. The yield of pure, white product is at least 80%. The formation of by-products can occur in the sulfonation reaction or the neutralization; particular mention may be made of the hydrolysis of the complexing agent as secondary reaction which contaminates the product and at the same time makes the economics of the process questionable as a result of the loss of material.